This invention relates to a novel method of detecting the vaporization of getter material during the manufacture of a CRT (cathode ray tube). In addition to detecting the vaporization of getter material, the method can be used to determine whether the vaporization is predominantly exothermic or endothermic, whether more than a prescribed quantity of getter material has vaporized, and whether a substantial amount of ionization has occurred during the vaporization.
In one popular design of a color television picture tube, which is a type of CRT, a getter container having getter material therein is held against, or close to, the inner surface of the envelope, usually that part of the envelope called the funnel. After the envelope is evacuated of gases and sealed, an induction coil is positioned against, or close to, the outer surface of the envelope opposite the getter container and is then energized with high-frequency current. The magnetic field generated by the energized coil induces currents in the getter container, causing the temperature of the getter container and the getter material therein to rise rapidly to temperatures above about 800.degree. C. until the getter material, which is usually barium metal, vaporizes and deposits as a getter film on the internal surfaces of the tube. A purpose of the getter film is to absorb both residual gas left in the envelope after evacuation and to adsorb gas that is later evolved from internal surfaces during the operating life of the CRT. The life of the CRT is determined principally by the ability of the getter film to continue to absorb gas and to maintain a low gas pressure in the envelope. Thus, it is desirable to deposit an optimum distribution of the greatest amount of getter material in the CRT.
With the demand for higher quality and lower manufacturing costs, it is important to know, during or soon after the induction coil is energized, whether vaporization has taken place. Also, it is desirable to know some of the qualities of the vaporization, such as whether the vaporization is predominantly exothermic or endothermic, whether more than a prescribed quantity of getter material has vaporized, and whether a substantial amount of ionization has occurred during the vaporization. Prompt knowledge of one or more of these factors permits prompt adjustment of the equipment, particularly automated equipment, and permits tubes which have not been processed properly to be identified and removed for reprocessing.
At present, two methods are used to determine whether vaporization has taken place. One method takes advantage of the light that is emitted by the container and getter material during the heating step. An operator observes the change in the brightness of light emanating from inside the CRT during the energization of the coil. This method is only qualitative and may be obscured by structures, such as metal shields, in the CRT. In the other method, the operator observes whether metal is deposited on a clear area of glass of the CRT envelope. That clear area must be in the path of the vaporized material and the path must not be obstructed by structures, such as metal shields inside the CRT. By either method, the operator must take time to observe the event, and then can determine only whether vaporization has taken place if all of the above-mentioned conditions are satisfied.